The most common stepper motor systems employ special two phase synchronous motors in combination with a phase current switching system. The basic full step sequence is achieved by energizing phase A with a positive signal, then energizing phase B with a positive signal as phase A is turned off, then energizing phase A with a negative signal as phase B is turned off, then energizing phase B with a negative signal as phase A is turned off, and then repeating the sequence. The rotor of the motor is advanced incrementally by this four-step sequence, taking one full step at each change in the phase currents. The motor itself preferably includes a large number of poles so that one revolution of the motor includes a large number of incremental steps. An example of a suitable motor for such stepper applications is disclosed in U.S. Pat. No. 4,330,727.
More sophisticated stepper systems employ a control technique called "microstepping" where the motor can be controlled for positioning at a variety of positions intermediate the normal full step positions. Here, the number of intermediate positions is determined by the drive system controller. Such intermediate positions are achieved by proportioning the signals applied to the phase A and phase B windings to obtain a field vector as required for each intermediate point. Since the effect of the control is basically analog rather than digital, the presence of harmonics in the torque characteristic have a substantial effect on achieving position accuracy and, hence, are of great concern.
Various techniques have been employed in the past for suppressing harmonics of the characteristic torque in electric motors. U.S. Pat. No. 4,518,883 describes one of the most effective prior systems wherein a selected number of torque harmonics could be eliminated by following a prescribed procedure for shifting the torque producing stator poles from their nominal positions. In so doing, symmetrical groupings of poles were chosen and then shifted or rotated relative to each other by the angle Pi/H, electrical degrees, where H is the harmonic being suppressed. The number of harmonics that can be eliminated with this technique depends upon the number of independent symmetrical groupings found in the motor. If the number of independent stator poles is N=2", then the technique of symmetrical shifting of pole positions can be used to substantially eliminate U harmonics.
The disadvantage of the technique described above is that the number of harmonics that can be suppressed depends upon the number of symmetrical groupings existing in the particular motor design. Also, the design can result in an undesirably large loss in motor torque output.